This read-only field reports the ifIndex value the agent automatically assigned to this circuit when its ciCircuitStatus became active, linking the circuit-specific table back to the standard ifTable/ifXTable used for generic interface statistics. An admin monitoring circuit performance uses this value to know which ifIndex to poll for the familiar ifInOctets/ifOutOctets/ifOperStatus counters that correspond to this specific circuit. For example, if this object returns 42, the admin would query ifTable.ifEntry.42 to get the standard interface counters tied to this particular circuit.